1. Field of the Invention
The present invention relates to a printing apparatus and a sheet cartridge, and more specifically to a mechanism that feeds printing sheets in a printing apparatus such as a printer or a copying machine.
2. Description of the Related Art
Conventional sheet feeding mechanisms of this type are provided with a mechanism for properly maintaining a conveying position or posture of a sheet during sheet feeding. The mechanism enables the sheet to be prevented from slant moving so that printing is executed to an accurate position of the sheet during printing operation, which accompanies conveying of the sheet after sheet feeding.
For example, in a known sheet feeding mechanism of a printer, a pair of guide members is fixedly provided so that the distance between the guide members is slightly larger than the width of sheets. This arrangement serves to regulate the sheet feeding direction or position of the sheets. Further, in another known form, one of the guide members can be moved depending on the size of sheets, while the sheets can be pressed against the other guide member. Furthermore, as described in, for example, Japanese Patent No. 3180819, a copying machine provided with a tamper is known wherein when sheets are stacked on a feeding section, the tamper biases the sheets in a direction crossing the sheet feeding direction and presses the sheets as in the case with the above guide members.
However, the size of sheets such as their width may vary slightly. In this case, if the guide members are fixedly provided as in the above conventional example, looseness may be created between the guide members and the sheets in accordance with the variation of the sheet width. Consequently, the sheets may be inclined in the conveying direction of the sheet.
Further, also in the form in which the guide member or the damper is pressed against a sheet, the guide member may be shifted because of the weight of sheets depending on the position of the printer because no urging force is basically exerted after the pressing. As a result, the sheets are inclined when fed. For example, if a sheet cartridge (cassette) is installed in a relatively small-sized portable printer, the load a few or several times the weight of sheets due to a vibration or a shock caused by carrying of the printer may act on the guide members, depending on the position of the printer when it is carried. Thus, the guide members may be shifted to incline the sheet feeding direction.
To solve this problem, it is considered that a spring may be used to urge and press the guide members against sheets. However, in a configuration in which this urging force always acts on the sheets, the sheets may be deformed, e.g. curled. Consequently, the relative positions of the sheets and, for example, a print head may deviate from the original ones to reduce the accuracy of a print position. Further, if this urging force is allowed to act during printing, a relatively large force is required to convey fed sheets. This creates problems such as an increase in the power consumption of a relevant motor and as degradation of print quality due to an increase in a slip amount of the conveyed sheet.